Hi Living. When you tell someone their B/F., husband, or significant other is cheating then you will probably be the bad one. You will be the liar!! At least that is the way the story usually works out.
Maybe I am not clear, but it seems that the dancer told other people about this cheating.....it does not sound like she told you straight out. Important rule always in life. If you did not see with your own eyes you are not holding anything back!!!!! You know nothing!! And that is the truth
If...you find out for a fact, with your own eyes that this man has cheated on your sister......if it was me....I would go to this jerk and tell him what you know. I would tell him he better tell your sis whats up!!!